MITOSIS
Mitosis - The division of cells to produce genetically identical daughter cells.
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Happens in multi-cellular organisms
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Division and replication of nuclear material
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Has 2 phases:
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INTERPHASE - Normal Cell Activity
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MITOTIC PHASE - Division of cell
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IMPORTANCE OF MITOSIS
a) Reproduction (e.g. amoeba)
b) Growth & Development (e.g. embryos)
c) Tissue Renewal (e.g. RBCs)
Genome - Complete genetic information of an ORGANISM (packaged in chromosomes).
Chromatin - Unorganized form of DNA (loosely assorted DNA threads).
Nucleotide - Phosphate + sugar + base
Nucleosome - Every 200 nucleotide pairs, the DNA wraps itself twice around a group of 8 histone proteins.
Coiling - DNA coils around protein
Super-coiling - Nucleosomes start coiling around each other
*** Chromatin condenses and coils around protein to form chromosomes during replication.
PROKARYOTES
- Single strand of DNA
- Single stranded
EUKARYOTES
- 1000x more DNA strands than prokaryotes
- Double stranded
HUMANS
- 46 chromosomes
- 23 nearly-identical homologous pairs
(1 from each parent)
CHROMOSOMAL PAIRS
22 - Autosomal Pairs
1 - Sex Pair
KARYOTYPE
Figure 1. Chromosomes before and after replication
Figure 2. Chromosomes & chromatids
Homologous chromosomes
- Look alike
- Control the same traits
- Inherited from each parent
- May code for different forms of each trait.
Non - Homologous chromosomes
- Look different
- Control different traits
SEX CHROMOSOMES
- Distinct from each other in their characteristics
- Represented as X & Y
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XY - Male
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XX - Female
- In a diploid cell (normal functioning cell - 2n), the chromosomes occur in pairs. The 2 members of each pair are called homologous chromosomes or homologues.
2 disks of proteins called kinetochores are bound to the centromere. Microtubules attach to these kinetochores and pull them away during mitosis.
Faults in kinetochores lead to mutations and faulty cell division (chances increase with age).
Figure 3. Chromosome & Kinetochores
Process of Mitosis
INTERPHASE
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G1 - Increase in cell size and mass (to adult cell)
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S - Duplication of DNA
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G2 - Replication of all organelles
THE MITOTIC SPINDLE
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Aster - Connected to cell membrane, radial array of microtubules from centrosomes.
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Centrosomes [2 pairs of centrioles] (microtubule organizing center)
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Spindle microtubules
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Kinetochore microtubules - attach to chromosome
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Non-kinetochore microtubules - attach to opposite incoming microtubules.
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CYTOKINESIS
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Cleavage forms and two daughter cells are formed
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IN PLANTS - Cell wall forms between the two daughter cells at the 'cell plate'.
PROPHASE
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Nucleus increases in size
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Early mitotic spindle fibre forms
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Nuclear envelope disintegrates (prometaphase)
METAPHASE
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All chromosomes align in center of cell at metaphase plate
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All spindle fibres attach to chromosomes/other spindle fibres
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Each chromosome must be attached to microtubules of both centrosomes.
ANAPHASE
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Chromosomes are pulled apart
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Kinetochore microtubules shorten and constrict, pulling chromosomes to two ends of cell.
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Non-kinetochore microtubules elongate and cause cell to elongate.
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Cell opens up from center and starts squeezing from center.
TELOPHASE
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Nuceli of cell starts to develop from fragments of broken nuclear envelope of parent cell
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Mitotic spindle fibre breaks down
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Nucelolous reforms first
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Chromosomes become chromatim
Figure 4. Mitosis
IMPORTANCE OF CYTOSKELETON IN MITOSIS
1. Microtubules - Controlling movement of chromosomes and allowing cell to elongate.
2. Intermediate filament - Allows cell to elongate and hold organelles in place.
3. Actin filament - forms constriction belt and allows for cleavage furrow.
APOPTOSIS - How cell self-destructs (lysosomes)
*If DNA is damaged, cells lose ability to control rate of cell division (mutation)
*Cell division at an uncontrolled rate is cancer
TUMORS
Benign - Not cancerous, do not spread to other parts of body.
Malignant - Canerous, cells break lose and can invade and destroy healthy tissue in other parts of body (metastasis - when cancer infects healthy tissue)